swift234

Revert all the function type ABI restriction changes.

John pointed out that messing with the type checker's notion of "subtype"
is a bad idea. Instead, we should just have a separate check for ABI
compatibility...and eventually (rdar://problem/19517003) just insert the
appropriate thunks rather than forcing the user to perform the conversion.

I'm leaving all the tests as they are because I'm adding a post-type-checking
diagnostic in the next commit, and that should pass all the same tests.

Part of rdar://problem/19600325

Swift SVN r25116